It just works. I haven't experimented a lot with the built-in microphone because I have been in enough meetings that have been ruined by people using the built-in microphone. But I never heard any complaints about echos, and when I record myself using the speakerphone the audio quality is fine. I have also experiemented with background noise such as typing, and as I long I don't speak, I can't hear myself typing.
The mic and speakers on the laptop are designed for a single listener/speaker sitting directly in front of the machine.
A USB speakerphone is designed for multiple speakers and a wider area in which they could be positioned.
You also get some advantages from isolating the mic/speaker from the laptop chassis, noise control and such. Same reasoning as using an external DAC/Amp for headphones rather than the onboard interfaces.
